I have been having the same problem with my 93. When I torque the motor the EM flange pushes against the downpipe and it opens at the bottom to give me the pft pft pft sound. I tried an exhaust shop that told me they didn't have a big enough donught gasket for it. So I bought one from the dealer and it is bigger but I still have the leak. I will watch this thread closely to see what you find out. My only next step was a new downpipe that would hopefully be the correct angle to stop this leak.

I kinda hope I'm having the same problem, as I want to believe the flex is OK :?

My very next step is to act as if there is a leak between the rear EM and DP, to rule that out.

My '98 had no donut between the EM and the DP...looking through parts catalogs, it seems the '97 and earlier had it:
http://www.napaonline.com/masterpage...st+Pipe+Gasket

Maybe I should get one of these?

On the other hand, I wonder if it would be a good idea to stick a huge ring of exhaust sealant on the EM (with or without the metal gasket???), as pictured by the red ring (kind of like sealing a coolant neck with RTV):


Opinions, anyone?

The shop that I went to that didnt have a gasket put that sealer on it and it lasted about 20 miles and that was it. Sorry, but that will not solve the problem there because there is too much movement in that area. It is made to move around due to the motor torque with it being mounted sideways. I would for sure try the gasket first in your case. I know how you feel though I hate exhaust leaks and I can't get rid of it without a good exhaust shop and some money. :(

Yeah, that sealant dries to something quite brittle, so I can see where the ring o' sealant idea won't cut it.

I can confirm now that there is a leak between the rear EM and DP flanges...this morning on cold startup, I saw some moisture bubble up out of that joint.

It looks to me like in 1998, GM switched from a donut to an embossed gasket to seal that joint. The rear EM is interchangeable from '97 to '98, and the flanges on this 2.5" DP look to be recycled from a stock DP, so I'm thinking that the stock '97 donut I linked to earlier might do it. It'* worth $10 to find out.

The CEL came on during my ride to work, so I guess the PCM figured out the rear O2 is gone. I'll have to program that out.

my 97 doesn't have a doughnut there, flat metal gasket with a lead ring.

The napa catalog shows a flat gasket for '98 LeSabre:


But a donut for '97 LeSabre:


Maybe the switchover year for Bonneville was earlier?

Anyway, $11 later and I've got the donut (FEL-PRO 60640):


It is a tapered ring measuring 3.02" OD at the fat end, 2.78" OD (2.51" ID) at the skinny end, and 0.58" thick (not counting a small lip on the fat end)

I'm assuming the fat end goes into the rear EM, and the skinny end goes into the DP. I guess I'll find out this evening .

The bonne and Riv changed over in 97. 96 and prior were donuts.

Ben had a leak(just one of many he has ) just like yours. I think he fixed it with a new flat metal gasket. I don't know, but I think that gasket may be directional(based solely off of the way the used one from Ben'* car looked. visual comparison of the inner ring vs. which way seemed to sit best in the DP flange). Just a thought? Those two flanges shouldn't be moving I wouldn't think. That'* the job of the flex pipe.

Take all of this with a grain of salt. We're comparing a 97'* to 98'* and it'* already mentioned that they changed over.

the little lead ring on the flat gasket has to go to the dp side

The tapered part of the gasket goes to the DP. I just want to note incase anyone is following this with the same problem. I do not have a flex pipe on mine. This is why there are springs on the flange bolts, a ball type of gasket, and a flared end to the DP to allow for movement when the engine twists back against the solid exhaust. My DP is just enough off that it opens up under heavy acceleration and the flanges meet at the top. I could probabally have a shop cut it off and bend it but I don't have the money for that right now.